Peras



`lune 14, 1960 1 PERAs Low-TENSION SPARK PLUGS Filed April 1, 195e .nited States atf LOW-TENSION SPARK PLUGS Lucien Pras, Billancourt, France, lassigner to Regie Nationale Ydes Usines Renault, Billancourt, France Filed Apr. 1, 1958, Sen'No. 725,617

Claims priority, application .France Apr. 29 .1957

Claims. (Cl. 313-131) This invention relates to spark plugs and has specific reference to improvements in low-tension spark plugs for internal combustion engines which operate by the discharge of a condenser.

It is already known to constitute the sparking elements of low-tension spark plugs utilized for igniting the fuel mixtures of internal combustion engines by the discharge of a condenser in the form of two electrodes spaced about .004 to 0.02 apart, the gap therebetween being filled by an insulating member on which the spark is caused to slip.

Now it is the speciiic object of this invention to provide a specific arrangement of the various component elements comprising the sparking electrodes and the sparksupporting insulator, whereby a regular, reliable operation may be expected for a relatively long time period, for instance in internal combustion engines.

With this arrangement, the insulator of the spark plug body constitutes at one end the spark-supporting insulating element, a rod-like conducting member having a headforming widened end portion constituting one of the electrodes, said conducting member extending axially through said insulating element.

In order to aiord a clearer understanding of the invention a spark plug made according to the teachings of this invention is shown diagrammatically and in axial section in the single figure of the accompanying drawing.

Referring to the drawing, the spark plug shown therein comprises two electrodes 1, 2; the first or central electrode 1 consists of a disk of refractory metal such as tungsten or thoriated tungsten, and the other electrode 2 consists of a hollow cylinder having a tapered bore and is also made of refractory metal; this other electrode 2 is supported in the spark plug body 4 and locked in the proper axial position by an inner ange 4a formed at the lower end of the spark plug body.

This annular electrode 2 has tted in its tapered bore the correspondingly tapered lower end of the insulator 3 of the spark plug body which may project slightly from the outer end or face of the electrode.

The outer diameter of the electrode 1 is of the same order of magnitude than the bore diameter of electrode 2; it may even be slightly greater than this bore, for example 0.004, or equal, if the spark-supporting insulator projects from the base of the electrode 2 on the sparking side, or smaller if the insulator end face is co-planar with the base of the electrode 2 in the sparking zone. The insulator 3 consists of a ceramic member, for example of silicoaluminous ceramic having a high (70 to 95%) alumina content, and is snug lit in the spark plug body consisting of two metal members 4 and 5. A current lead-in rod 6 extends axially through the insulator 3 and terminates on the electrode end with a head-forming widened portion 8 engaging with its inner face the electrode 1, the opposite end of this rod consisting of a screw-threaded portion connected to the ignition distributor. The central rod 6 is sealed in the insulator by means of an adequate cement. A pair of copper or like relatively soft washers Patented J une. 1v1'4, 1960 Y7 vare provided to seal the spark plug against'combustion gas leaks.

The different component elements of the spark plug are assembled as follows:

'The outer electrode 2 is inserted in the metal 'body 4 and held therein through `any suitable means. The 'insulator 3 is then fitted and held in position Aby locking the nut 5. The two joint-forming sealing washers I are disposed as 'shown to seal the assemblywhile'permit'ting Van eicientlocking'of the parts without any'riskfof breakling the insulator. The -crushing of the copper -orilike washers is stopped when the insulator properly contacts the surface of the tapered bore of electrode v2 with its outerperipheral surface.

On its electrode or sparking side the insulator ma either project slightly from the plane of the outer face of the electrode 2, or be level or flush therewith; the corresponding adjustment is made by using a suitable jig and grinding the parts with the component elements of the plug in their assembled condition so far described. If desired, the insulator may also be slightly recessed relative to the base plane of electrode 2; in this case, the insulator may be ground before fitting it in the plug body.

Then, the electrode 1 is threaded on the steel rod 6 until it engages the inner face of the head 8, whereafter the rod 6 is threaded through the central hole of the insulator 3, cemented therein and secured by a nut screwed on a washer.

The electrodes 1 and 2 are not compulsorily of cylindrical shape with a circular cross-section, as this section may have any shape consistent with the specific conguration and characteristics of the combustion chamber. The electrodes may also have a prismatic configuration.

The cost of the electrodes may be reduced if desired by welding a thin flat ring of a refractory metal on a steel body, instead of using solid refractory metal alone.

The insulator end adjacent to the electrodes may be coated with a special enamel adapted to promote the pas sage of slipping sparks. This enamel is applied on the insulator before assembling the parts; after this enamel has been baked and reduced, the insulator end is ground in a plane at right angles to its axis in order to provide a at seat for the electrode 1 to be fitted thereagainst.

I claim:

1. In a low voltage surface discharge spark plug, in combination, means forming a tubular body, a tubular rst electrode disposed concentric with said tubular body and at one end thereof, the first electrode having atapered axial bore converging toward an end corresponding to an exposed sparking end surface of said electrode, an elongated, tubular insulator disposed extending axially in said electrode and said body, an annular disc second electrode disposed coaxial with said insulator at an end thereof corresponding to said one end of the tubular body and having a sparking surface spaced axially and radially inwardly of said sparking end surface of the first electrode, and an electrically conductive member extending axially throughout the length of said body through said insulator and the second electrode and having a head at one end thereof for holding said disc electrode snugly between said head and said insulator, the insulator having a sparksupporting surface bridging a space between the sparking surfaces of the two electrodes, and the insulator having a radial thickness adjacent the sparking end of the first electrode greater than the radial thickness of the first electrode.

2. A low voltage surface discharge spark plug according to claim l, in which said sparking-supporting surface of the insulator is an end face coplanar with said sparking surface of the rst electrode and in which said annular disc electrode has a transverse dimension smaller than said end face of the insulator.

Ytending axially outwardly o f said tubular electrode at said end to which said bore converges, and in which said annu- ,lai-,discz electrode has a transverse dimension equal-to a transverse dimension of said insulator portion, and Vsaid portion corresponding in, dimensions jminimuin 'dimensions of said bore.

Y4. A low Yvoltage surface discharge spark plug accord.

ing to claim 1, in which insulator has a portion extending axially outwardly of'said tubularl electrode at said end Yto which said bore converges, said portion corresponding in dimensionswith minimum dimensions of said bore,

kand in which said annularA disc electrode has transverse dimensions greater than said portion transverse dimen- -sions.V Y e Y 5 A low voltage surface discharge spark plug accord- I ing to claim l, in which said tubular body comprises an f 4 Y y internally threaded tubular body portion and an externally threaded tubular portion coaxial with said first tubular body portion, and in threaded engagement therewith and relatively movable axially thereto, said insulator having a circumferentially disposed integral flange portion clamped between said two body portions.

References Cited in the-lille of this patent UNITED STATES PATENTS 11,272,954 Horning r July 16, 1918 Y 1,543,628 Stover June 23, 1925 2,527,489 Smits Oct. 24, 1950 2,760,099 Suter Aug. 21, 1956 2,831,138 Teasell Apr.'l5,V 1958 FOREIGN PATENTS Y 22,517 Great Britain Sept. 28, 1910 

